The present invention relates to a process for heating slabs, billets, blooms and like metallic objects, and to a furnace which can be utilized for the practice of the process. More particularly, the invention relates to improvements in reheating furnaces and to a process for treating metallic objects in such furnaces.
The trend in the steel industry is toward increased synchronization between continuous casting machines and rolling mill trains. This renders it possible to admit slabs, blooms, billets and like objects which issue from a continuous casting machine directly into a rolling mill train or to admit such objects into the reheating furnace in a condition in which they require a minimum of reheating, i.e., the temperature of each object should be relatively high at the time the object enters the reheating furnace. The result is a pronounced reduction of energy requirements for reheating of the objects prior to introduction into a rolling mill.
The above outlined prerequisites can be met only if the construction and mode of operation of a reheating furnace are attuned to the requirements of the machines which supply the objects as well as to the requirements of machines which process the objects subsequent to reheating. Thus, a modern reheating furnace should be capable of reheating cold objects as well as of merely increasing or reducing the temperature of an object which is delivered at a temperature below or at a temperature above the optimum rolling temperature, e.g., at a temperature between 1100.degree. and 1260.degree. C. In addition, a modern reheating furnace should be capable of storing a certain number of objects so as to take into account potential fluctuations in the output of a continuous casting machine and/or potential fluctuations in the requirements of a rolling mill train, i.e., of storing a certain number of objects when the output of the casting machine exceeds the requirements of the rolling mill train as well as of satisfying the requirements of the rolling mill train when the output of the casting machine drops below a standard value.
As a rule, a billet, a bloom or a slab must be heated primarily in the region which is adjacent to its external surface because the temperature of the cores of such objects deviates rather slightly from the optimum rolling temperature (it is normally between approximately 1100 .degree. and 1260.degree. C.). Thus, only the stratum or strata which are close to the external surface of such an object are likely to undergo pronounced cooling during travel from the casting machine to the rolling mill train. For example, a slab comprises a core which constitutes the major portion of the slab and whose temperature is optimal for immediate rolling or such temperature even exceeds the optimal value so that only the relatively thin outer portion or stratum of the slab requires a reheating preparatory to admission into a rolling mill. The reheating furnace between the casting machine which turns out slabs and the rolling mill train must be designed with a view to ensure adequate heating of the outer stratum or strata of each slab but without overheating the core whose temperature already matches or very closely approximates the optimum rolling temperature.